The present invention relates generally to bioreactor systems and in particular to a bioreactor system capable of continuous mode production.
The use of biocatalyst for the production of useful products has been a part of man's history for thousands of years. Until this century the production of useful products from biocatalysts was performed in batch-type reactors (fermenters) which for the most part use agitation as a means of intimately mixing the contents of the reactor and adding essential gases (i.e. oxygen, carbon dioxide, nitrogen, etc.) through some form of sparger. Furthermore, as man's basic knowledge of biological systems grew (i.e. pH, oxygen, sterility, nutrition, etc.), he began to utilize this information for the purpose of improving the quantity and quality of biologically derived products for commercial exploitation.
The basic economics behind biological production are two-fold: producing a product with greater value than the raw material it was derived from and producing a product that cannot be economically made in any other way, in both cases for the purpose of satisfying market demand.
For bulk biochemical production, such as beer, the principle costs are in the initial raw materials and initial capital costs for the production facility since millions of gallons of beer must be producted to obtain a reasonable return on the investment. It should also be noted that yields of these products are some of the highest of any biologically derived products.
At the other extreme is high-value biochemical production, such as monoclonal antibody, whose largest expense is in the downstream processing of the product. This occurs because of federal regulations stipulating the purity of the final product if it is to be used in vivo. In this situation, however, the very small amount of product produced and the difficulty in performing the production requires that the final product be sold at many orders of magnitude higher than the bulk biochemical for the same basic quantity.
Biochemical production is still today, for the most part, a batch operation. Many other industries such as oil and plastics have converted to continuous operation since continuous production typically requires much smaller capital investment to produce the same amount of product and one can either increase or decrease production to better meet market demand.
A number of bioreactor (fermentation) systems and processes have been invented for the purpose of bringing commercially usable, continuous production to the biologically-derived fermentation industry. To be a truly commercially-usable system, however, the system must substantially reduce the overall manufacturing and capital costs of the product to the producer at the same or higher yields since these are the producers incentives to make the investment in the system. A total overall manufacturing cost reduction related to the hardware can only occur by a continuous system that reduces both up-stream and down-stream unit operation costs and/or by increasing the overall yield of the system through regulating environmental and other parameters.
Another important consideration is the energy and mass conservation of the system. In a batch type of system, a considerable amount of energy and time is spent making the biocatalyst (mass). This is seen in the traditional batch growth curve during lag and log phase. Once the batch has been completed, however, the biocatalyst is then wasted. This usually coincides with stationary and/or death phase. Furthermore, when the up time and down time cycles of batch type systems are added into the equation as well as the tremendous loss in material should the system ever become contaminated, the real expense of batch systems in time, energy and mass becomes quite excessive. Almost all research into biocatalyts growth, behavior, and productivity has been done in batch systems since these were the only systems available. In recent publications it has been shown that biocatalyst can be maintained for many months without the loss of productivity. New continuous systems attempt to prolong stationary and/or maintenance phase for as long as possible, since the biocatalyst's life span, if handled correctly, can be extended to many months. If growth and productivity can be managed for long periods of time, then the initial energy and time required to create the biocatalyst (mass) becomes trivial over the length of a continuous system. However, unless the continuous system is able to manage growth and productivity without utilizing excessive resources, the continuous system would be as expensive as the batch system and offer no great advantage. Therefore, every unit operation of the continuous system must be optimized in such a way as to utilize the least amount of resource to obtain the desired results and still be able to manage the growth and productivity of the biocatalyst. As used here, the term biocatalyst or mass will be understood by the skilled artisan in this field to include enzymes, cells or other biochemical catalyst.
Prior to the present invention, the prior art relating to this invention may be classified into two major hardware system catagories, which are recycle and immobilization. None of the prior art integrates into its hardware or process, capabilities or those important characteristics mentioned above.
A recycle system typically consists of a standard batch bioreactor which may be stirred and/or contains a sparging device and where the biocatalyst and other contents are pumped out of the reactor and then through some form of separator (i.e. membrane, centrifuge, dialysis, etc.) for the purpose of separating the product from the biocatalyst. What remains after the product has been separated is then pumped back into the reactor to be reused. The obvious intent is to reduce downstream processing costs by incorporating the separation step as part of the fermenter. The advantages to this type of system include the ability to agitate and aerate the biocatalyst directly for better mass transfer, separation of the product (if it is extracellular), and ease of monitoring the environment of the biocatalyst. The disadvantages are:
1. Pumping the biocatalysts can damage or destroy them and thereby nullify the benefits obtained from the built-in separation step by adding additional components to the product stream, releasing protieolytic enzymes from a biocatalyst that would damage or destroy the remaining biocatalyst, and where membranes are used, fouling would occur faster. Monitoring probes would also foul faster with the addition of further debris. This would also greatly increase downstream process costs in high-value biochemical production to reach the required purity levels.
2. The probability of contamination of the biocatalysts by having to sterilize and move the biocatalyst through additional is increased.
One example of a recycle system utilizing dialysis for separation is that of U.S. Pat. No. 3,186,917 issued Jun. 1, 1965 to Gerhardt et al. In one configuration, Gerhardt et al utilize a standard fermenter whose contents are circulated through the right-end compartment of a three compartment chamber by pumping. The compartments are separated by dialysis membranes. Dialysate is circulated through the middle compartment while feed circulates through the left-side compartment both by pumping. By regulating the flow of the various recycling fluids, product is extracted by the dialysis and feed passes through the dialysis compartment into the recycling biocatalysts fluid. Gerhardt et al controls the product production and product extraction by the volume of dialysis fluid and the rate at which it is circulated.
One other illustration of a recycle system is Japanese patent 57-166985 of Oct. 14, 1982 to Kenkyusho. In his system, Kenkyusho uses the same approach as Gerhardt et al even down to the same three compartment chamber. The structural differences are that Kenkyuso uses two chambers which are interconnected and in place of dialysis membranes, microporous membranes are used instead. Kenkyusho recycles the biocatalyst through the middle compartment and sends the other liquid through one-side of the first chamber and exits through the other side of the chamber to enter the second chamber where the same thing occurs. The product is separated by recycling the separated product stream back through the recycling biocatalyst stream. Aeration, agitation and monitoring all occur within a standard batch fermenter used to hold the biocatalyst.
Both of these systems utilize a standard batch fermenter with extra tubing to make the systems continuous. Yet both these systems designs lack the ability to easily control the biocatalysts growth and activity.
An immobilized system typically consists of a biocatalyst that is immobilized onto a stationary surface. In an immobilized system, the biocatalyst cannot be directly moved and is not moved nor generally, is direct aeration applied to the biocatalyst. The stationary surface could be a membrane, sheet of plastic, or even a sheet of glass. The basis of an immobilized system is to utilize diffusion as the means of bringing and removing material to and from the biocatalyst, attempt to duplicate the cardio-vascular system, and to separate the biocatalyst from the product. Typically, a large reservoir holds the circulation fluid. It is in this reservoir that aeration would take place by some other device, but not where the biocatalyst is located and where the environmental variables are measured. The circulation fluid is either very slowly passed over the biocatalyst or flows on the other side of a membrane holding the biocatalyst. These systems are generally called perfusion reactors. This is because they rely on diffusion and act as a plug-flow reactor as the fluid travels over the biocatalyst or membrane by increasing in wastes and decreasing in nutrients. Instead of constantly circulating the biocatalyst as with recycle, the immobolized system constantly circulates the feed past the biocatalyst. These types of systems are generally much simpler and require less capital investment. The advantages of this system are that shear sensitive biocatalyst and slow growing biocatalysts can be easily grown. Also, as anchor dependent biocatalysts, are simple to set up and operate. This system can also easily be computer controlled/automated, and separation is possible.
The disadvantages of this system are:
1. Inability to directly agitate or aerate the biocatalyst;
2. Inability to directly monitor and, in some cases, even visibly see the biocatalyst;
3. Inability to control the growth and activity of the biocatalyst;
4. Seeding the reactor;
5. Many scale-up problems; and
6. Sterilizability, since most units are completely made of plastic.
An example of an immobilized perfusion system is U.S. Pat. 3,734,851 issued May 22, 1973 to Matsumura. In his system, Matsumura entraps biocatalysts between two dialysis membrane sheets, forming a membrane bag. Feed is circulated over the one side of the membrane bag while another stream circulates on the other side of the bag. The biocatalyst is immobilized (entrapped) within this membrane bag. There is no agitation nor direct aeration possible for the contents of the bag nor can the biocatalyst be directly monitored or visibly seen. A number of these bags are placed on top of one another with appropriate channels between them.
Recycle and immobilization systems both have certain advantages and capabilities inherent in their respective design. The advantages of one however, are for the most part, the disadvantages of the other.